1. Field of the Invention
The present invention relates to an organic electroluminescent device (OELD), and more particularly, to an organic electroluminescent device that uses a phosphorescent material as a luminescent material.
2. Discussion of the Related Art
Recently, with the trend of a large sized display, a request of a flat display that occupies a small area has been increased. One example of the flat display is an electroluminescent device. The electroluminescent device is divided into an inorganic electroluminescent device and an organic electroluminescent device.
The inorganic electroluminescent device is commercially used and emits light in such a way that electrons are accelerated by applying high electric field to a luminescent portion and the accelerated electrons collide against the center of the luminescent portion to excite the center of the luminescent portion. The inorganic electroluminescent device has drawbacks in that power consumption is high and it is difficult to obtain high luminance and various luminescent colors.
On the other hand, the organic electroluminescent device has a sandwich structure in which a thin film shaped organic luminescent material having semiconductor characteristics exists between an anode and a cathode. The organic electroluminescent device emits light such as phosphor or fluorescence as excitons are transited from an excited state to a ground state. The excitons are produced by injecting electrons from the cathode and electron holes from the anode into the organic luminescent material if a direct current is applied to the cathode and the anode and by combining the injected electrons and the electron holes with each other. The organic electroluminescent device has high response time and is driven by a direct current voltage of several volts to several tens of volts. Also, the organic electroluminescent device can obtain high luminance of several hundreds to thousands of cd/m2 and various luminescent colors depending on the change of a molecular structure.
For commercial use of the organic electroluminescent device, it is necessary to improve efficiency of the luminescent device and enable full color display. To this end, the related art organic electroluminescent device has a multi-layered structure. That is, the related art organic electroluminescent device includes a material layer having electron injection (transport) and electron hole injection (transport) characteristics in addition to a luminescent layer formed between a cathode and an anode. The material layer serves to control injection quantity and mobility of the electrons and the electron holes, thereby obtaining high efficiency.
However, the related art organic electroluminescent device having a multi-layered structure as above has a problem in that it is not sufficient for display due to insufficiency in luminescent efficiency and lifetime of the device. Accordingly, to obtain an electroluminescent device having high luminance and high efficiency, development of the device is required in both a structural aspect such as adding a new layer and a material aspect such as developing a material having excellent luminescent efficiency or excellent injection and transport characteristics of the electron holes (or electrons).